1. Field of the Invention
The present invention relates to a method and an apparatus for creating vacuum used when a substrate is carried into and out of a process chamber and, more particularly, it relates to a method and an apparatus for making each of load lock chambers highly vacuous in the etching and CVD systems.
2. Description of the Related Art
In the course of applying the film forming and etching processes to semiconductor wafers, it is needed that the process chamber is kept highely vacuous because particles must be prevented from adhering to the surface of each wafer. The process chamber is exhausted by a vacuum pump and thus made vacuous, having a vacuum degree smaller than 10 Torr or preferably ranging from 10.sup.-3 to 10.sup.-9 Torr. In addition, load lock chambers are arranged around the process chamber to prevent atmospheric air and particles from directly entering into the process chamber. It is also needed that each load lock chamber is made clean and vacuous to same extent to which the process chamber is made so.
In order to make the load lock chamber highly vacuous, the load lock chamber is exhausted by rotary, dry and turbo molecular pumps. It is also returned atmospheric while filling it with nitrogen gas after the wafer is carried out of it. When it is repeatedly made atmospheric and vacuous in this manner, particles most of which have been stuck and piled on its inner walls float and adhere to the surface of the wafer.
On the other hand, various kinds of gas molecule such as water component, carbon oxide, carbon dioxide and nitrogen have adhered to the surface of each wafer and the inner walls of each chamber and these gas molecules are likely to release from the wafer and the inner walls of the chamber under depressurized state. In order to vacuum-exhausting the gas molecules including those released from the semiconductor wafer by the above-mentioned vacuum pumps, therefore, the time needed to exhaust the chamber becomes long and the chamber is not exhausted sometimes to a desired vacuum degree.
A trap device of the radiator type is used to make each load lock chamber highly vacuous. The conventional trap device has a passage which is commonly used by coolant and heat medium, and when gas molecules in the chamber are to be trapped, coolant is passed through the passage and when the chamber is to be returned atmospheric, coolant is discharged through it and heat medium is then passed through it. In the case of the conventional trap device, however, it takes a long time for coolant in the passage to be replaced by heat medium. In addition, the heat wearing of the passage material is caused and the life of the passage is thus made short because cooling and heating are repeated through the passage. Further, when air is supplied, as coolant, into the passage, water component in the air is condensed on the inner wall of the passage soon after the supply of the air is started. When the water has frozen solid, the passage is plugged by the ice. This prevents heat exchange from being achieved between the coolant and the trap device component.